It has been conventional in the petrochemical industry to scrub gases produced by cracking for ethylene production (or by other hydrocarbon conversion processes), with aqueous sodium hydroxide solution. Such a scrubbing treatment removes hydrogen sulfide and carbon dioxide primarily as sodium sulfide, sodium carbonate and sodium bicarbonate and also removes some of the higher molecular weight hydrocarbon constituents carried by the cracked gas stream. Discharge of such effluent would be environmentally harmful and at the very least neutralization is necessary. However, neutralization of the raw effluent involves regeneration of carbon dioxide and some of the hydrogen sulfide but still leaves a substantial content of hydrogen sulfide and sulfide ions which have a high COD. Processes have therefore been developed to oxidize the sulfide (and other inorganic sulfur acid salts) in the alkaline solution to at least thiosulfate ion and generally to the more environmentally acceptable sulfate ion, before neutralization. Such oxidation processes are generally Wet Air Oxidation (WAO) processes in which gaseous oxygen in the form of fine bubbles is contacted with spent caustic in contacting columns for relatively long residence times. The basic WAO process has been known for many years. Such a process which oxidizes to thiosulfate is described in Smith, A. G., "Ridding Process Waters and Caustic Solutions of Sulphides", Oil and Gas J, Jul. 9, 1956, p.95-99. Two articles by Otto Abegg in the publication Erdol und Kohle, Erdgas, Petrochemie (1) Vol. 14, No. 8, p.621-626 (1961) and (2) Vol. 15, No. 9,p.721-722 (1962) describe experiments aimed at complete conversion of sulfide to sulfate in the WAO process. Other descriptions of WAO processes are included for example in Martin, J. D. and Levanas, L. D., "New Column Removes Sulfide with Air", Hydrocarbon Processing & Petroleum Refiner, 41, May 1962, p. 149-153 and in U.S. Pat. No. 3,963,611 assigned to Chevron Research Company and U.S. Pat. No. 3,761,409 assigned to Texaco Inc. Various process variables are described such as temperature, pressure and steam injection as well as gas/liquid contacting devices and co-current and counter-current arrangements with and without catalysts.
Our earlier UK Patent No. 2,254,015 also describes a development of the WAO process.
Low pressure WAO processes, typically requiring only two stages of air compression to achieve 7-10 bars, can be more economical and reliable than WAO processes operating at higher pressures with more stages of compression. More complete oxidation therein is strongly favoured by higher temperature and residence time, and more contacting stages. Such processes have been carried out in a series of oxidation reactors, with liquid flow driven from one reactor to the next by keeping the reactor pressures at sufficiently different values. For a low pressure air supply, typically at approximately 7 bars, three reactors in series can be used in this way, but the resulting outlet pressure from the third reactor would be insufficient to drive the liquid through any additional reactors and their control valves.